1. Field of the Invention
The invention relates to a method and an apparatus for the contamination-free heating of gases.
2. Background Art
In the fluidized-bed deposition of highly pure polycrystalline silicon, a bed of silicon particles is initially introduced into a reactor, and this bed is fluidized by a gas and heated by a suitable apparatus to the temperature necessary for the deposition reaction. The silicon-containing compound present in the gas, in general a silane or a halosilane (SiHaX4-a, X=halogen a=1-3), decomposes on the hot particle surfaces in a pyrolysis reaction with formation of elemental silicon which is deposited on the surface of the silicon particles and leads to a growth in the size of the particles. The method can be operated continuously if continuously growing particles are removed as product from the fluidized bed and particles of smaller size, so-called silicon seed particles, are fed to the fluidized bed. Such a method is described by way of example in DE 199 48 395 A1.
In these methods, the net heat demand of the fluidized bed is for the most part obtained from the difference between fluidized bed temperature and temperature of the gases fed in, whereas the enthalpy of reaction of the gas-phase deposition is of minor importance. The fluidized bed temperature corresponds to the required reaction temperature for the pyrolysis reaction and depends to a greater extent on the type and concentration of the silicon-containing compound. With the use of silane or chlorosilanes, e.g. trichlorosilane, a silicon-free gas, such as, for example, hydrogen, is usually also used for dilution. The minimum fluidized bed temperature for the deposition reaction increases greatly, starting from silane, with increasing chlorine content of the silicon-containing compound, and is from about 500° C. to 1200° C.
It is beneficial to preheat the gases to the fluidized bed temperature before entry into the fluidized bed and thus to keep the heat demand for the fluidized bed low. However, such preheating in the case of silicon-containing gases or gas mixtures which contain these compounds is limited by the respective decomposition temperature of the silicon-containing gases. If the silicon-free diluent gas, e.g. hydrogen, flows separately into the fluidized bed, this could theoretically be heated even above a fluidized bed temperature in order to heat the fluidized bed thereby. Particularly at very high temperatures, however, there is the danger of potential contamination by the apparatus for preheating, both in the case of the preheating of the silicon-containing gas and in the case of the preheating of the silicon-free diluent gas. This is true, for example, with the use of metallic heating elements comprising, for example, tantalum, molybdenum or Kanthal alloy, as described in U.S. Pat. No. 6,827,786 B2, column 18, line 57 et seq., for the preheating of hydrogen to 1300° C. for the fluidized-bed deposition. Disadvantageously, contamination of the gas occurs as a result of the intimate contact with the surface of the heating elements. The surface of the heating elements consists of metal which contaminates the gas to be heated. In addition, further contamination of the gas to be heated occurs as a result of the impurities present in the metal of the heating elements. The degree of contamination of the gas increases with increasing temperature of the metallic heating elements.